The guidance on COVID-19 was that someone had been exposed to the virus if they had been within six feet of an infected person for more than 15 minutes, but the NFL discovered people testing positive for far less than 15 minutes or weren't within six feet of an infectious person. The NFL had the contact-tracing technology to prove it.

“That was a wake-up call,” said Dr. Allen Sills, the NFL’s chief medical officer. “We had to be more precise in our definition of high-risk close contacts because clearly transmission could occur outside those basic boundaries of time and distance.”

After an outbreak for the Tennessee Titans, the NFL changed its rules to further reduce close contacts for any length of time. It counted people as having been exposed to the virus if they had unmasked, indoor interactions with an infected person for any length of time.

“This has implications far beyond the NFL,” said Dr. Thom Mayer, the NFLPA’s medical director. “This thing is more transmissible than people thought.” 

The NFL found that the virus, in some instances, traveled farther than six feet, especially in small, poorly ventilated areas. Masks also seemed to matter a lot, more than the duration of contact. 

“We had to adapt and change our approaches,” said Jeff Miller, the NFL’s executive vice president who oversees health and safety.